U.S. Pat. No. 5,486,830 and U.S. Pat. No. 6,577,266, both of which are incorporated herein by reference, describe in-band tag systems in detail. In such systems, a small, battery-powered, radio-frequency transponder, or “tag”, receives radar pulses from a radar system carried on an aircraft or some other moving host platform. The tag modulates and amplifies the received pulses and transmits the modulated and amplified pulses back to the radar system. Such transmission of pulses from the tag back to the radar system is also referred to as the tag's echo. In some instances, the tag does not change the center frequencies of the received pulses when it transmits them back to the radar system. The radar system can interact with the tag in either a SAR (Synthetic Aperture Radar) or GMTI (Ground-Moving-Target-Indicator) mode. The radar and the tag are cooperable in various functional modes, including: (1) initial detection of the tag; (2) identification of the tag's type and specific identifying number or designation; (3) precise geo-location of the tag; and (4) communication of environmental and other valuable information, in digital form, from the tag to the radar. These modes are detailed in the aforementioned patents. The radar-responsive tags described above have many applications, including a variety of military and other U.S. government applications. In particular, they are applicable in areas such as situational awareness, combat ID, precision guidance and targeting, search and rescue, and friendly-fire incident mitigation.
During the above-described operation of the aforementioned radar tag technology, echoes from passive terrain return to the radar at about the same center frequencies as the tag's echoes, and at about the same times as the tag's echoes. These passive terrain echoes can thus interfere with the airborne radar's ability to detect the tag and decode data transmitted by the tag. This type of interference, also referred to as clutter, degrades the radar's ability to process the desired echoes coming from the tag
One method to suppress clutter is to impart an inter-pulse (i.e., pulse-to-pulse) phase code to the tag's echo, as described in aforementioned U.S. Pat. No. 5,486,830. The receiving radar system applies ‘azimuth’ or ‘Doppler’ processing to the received pulses. This latter processing by the radar system forms a so-called pseudo-image of the tag. The tag's signature in the pseudo-image is enhanced due to a reduction in the amount of clutter power present in the processed signal. The clutter suppression factor can be on the order of 10 dB or more, depending upon the design details of the radar, its antenna, and the velocity at which the radar platform moves as the radar transmits and collects echo pulses from the terrain and from the tag or tags in the illuminated scene.
It is desirable in view of the foregoing to provide for improvements in the suppression of clutter in radar pulses received by a radar system from a radar transponder.